Radiation images such as X-ray images have been widely employed in the medical field for diagnosis of diseases. As a method of obtaining the X-ray images, the so called radiographic method has been widely utilized, wherein a phosphor layer (or a fluorescent screen) is exposed to X-rays, having passed through a medical patient, that is a subject, to emit visible light, which exposes a silver halide photosensitive material (hereinafter also referred to simply as a sensitive material) in the same manner as in usual picture-taking, and thereafter a visible silver image is produced via development processing.
Recently, however, instead of an image forming method using a sensitive material incorporating a silver halide, a new method for directly capturing images from a phosphor layer has been proposed.
This method includes a method of imaging via fluorescence detection, wherein radioactive rays, having passed through a subject, is absorbed in a phosphor, followed by stimulating this phosphor, for example, via light or heat energy so as to emit radiation energy, accumulated in the phosphor via the above absorption, as fluorescence.
Specifically, a radiation image conversion method using a stimulable phosphor (hereinafter also referred to simply as a phosphor) is known (for example, refer to Patent Documents 1 and 2).
This method is one which employs a radiation image conversion panel containing a stimulable phosphor as follows: the stimulable phosphor layer of this radiation image conversion panel is irradiated with radioactive rays having been passed through a subject, resulting in accumulation of radiation energy corresponding to the radiation transmittance density of each portion of the subject; thereafter, the stimulable phosphor is stimulated via an electromagnetic wave (or an exciting light) such as visible or infrared light in chronological order to emit the radiation energy, having been accumulated in the stimulable phosphor, as stimulated emission light; and signals based on the intensity of the emission light are converted into electrical signals, for example, via photoelectric conversion, whereby the electrical signals are reproduced as a visible image on a recording material such as a silver halide photosensitive material or a display device such as a CRT.
The above reproduction method of a radiation image exhibits the advantage of obtaining a radiation image showing great detail information at a far lower exposure dose, compared to conventional radiographic methods employing a radiographic film in combination with an intensifying screen.
Since a radiation image conversion panel employing the stimulable phosphor accumulates radiation image Information, followed by emitting the accumulated energy via scanning exciting light, another accumulation of a new radiation image may be conducted after the scanning, resulting in repetitive use of the conversion panel Namely, while one radiographic film is consumed for each image in a conventional radiographic method, a radiation image conversion panel may be repeatedly utilized via this radiation image conversion method, resulting in advantages in resource conservation and economic efficiency.
Further, in recent years, a radiation image conversion panel exhibiting higher sharpness has been demanded. As a method of enhancing sharpness, various attempts to enhance sensitivity and sharpness have been investigated, for example, by controlling the form itself of the formed stimulable phosphor.
As one of these attempts, a method employing a radiation image conversion panel incorporating a stimulable phosphor layer, structured of elongated columnar crystals, has been proposed, wherein the elongated columnar crystals are formed on a substrate via a vapor growth method (also called a vapor deposition method) so that the crystal axis of the columnar crystals is inclined at a predetermined angle relative to the normal direction of the substrate (refer to Patent Document 3).
Recently, a radiation image conversion panel incorporating a stimulable phosphor has been proposed, wherein an alkali halide such as CsBr is utilized as a phosphor host and Eu is utilized as an activator, resulting in high X-ray conversion efficiency, which has not been conventionally realized.
However, in radiation image conversion panels used under a variety of conditions, adhesion between the substrate and the phosphor layer is one of the critical characteristics. To enhance the adhesion, there has been disclosed a method of placing a resinous sublayer containing a cross-linking agent between the substrate and the phosphor layer (Patent Documents 4-6). In cases in which only a resinous sublayer is placed, when forming the stimulable phosphor layer on the resinous sublayer of high surface roughness via the vapor growth method, poor adhesion to the substrate occurs and accordingly the crystal structure of the phosphor layer is unevenly formed, resulting in a tendency to cause varying sharpness and uneven graininess in imaging via a radiation image conversion panel. Further, temporal stability of the characteristics is likely to decrease because the film thickness of the resinous sublayer is too high.
Further, there may occur defects such as breaking of a radiation image conversion panel, peeling off the phosphor and nonuniformity of image when a curvature radius of a radiation image conversion panel is very small in case that the radiation image conversion panel put in a transportable container is irradiated with X rays and then it is subjected to be read with a reading system in which the radiation image conversion panel is bent in the reading system for reading information recorded in it.
(Patent Document 1) U.S. Pat. No. 3,859,527
(Patent Document 2) Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 55-12144
(Patent Document 3) JP-A No. 2-58000
(Patent Document 4) U.S. Pat. No. 4,563,580
(Patent Document 5) JP-A No. 2005-91222
(Patent Document 6) JP-A No. 2006-125854